Almost all of the heat at the Earth’s surface is the result of radiation from the Sun. A small fraction however – less than 1% - comes from below, in the form of heat from the inner layers of the Earth.

Although the surface contribution of this heat is low, if a drill could reach 50km or so deep (roughly the average crust thickness*) it would encounter temperatures high enough to melt rock.

Research teams have been attempting to accurately quantify the source(s) of this heat for many decades.

It’s now thought that there are two sources. One is the ‘Primordial heat‘ which is a remnant of the heat that the Earth had when the planet originally accreted . The other component comes from the radioactive decay of unstable elemental isotopes – notably Uranium, Thorium and Potassium.

Recent work has tracked high energy neutrinos coming from within the Earth (known to be the products of radioactive decay) and has focused on trying to quantify the amounts of heat produced by radioactivity.

Most research groups now assume that somewhere around 50% of the heat comes from radioactivity. The other 50% is therefore assumed to be ‘primordial’.

Chemical and physical Earth models agree little as to the radioactive power of the planet. Each predicts a range of radioactive powers, overlapping slightly with the other at about 24 TW, and together spanning 14–46 TW. Approximately 20% of this radioactive power (3–8 TW) escapes to space in the form of geoneutrinos. The remaining 11–38 TW heats the planet with significant geodynamical consequences, appearing as the radiogenic component of the 43–49 TW surface heat flow. The nonradiogenic component of the surface heat flow (5–38 TW) is presumably primordial, a legacy of the formation and early evolution of the planet."

Source : Reviews of Geophysics Volume 50, Issue 3

But explanations for the quantity of this primordial heat are severely lacking in detail - given the uncertainties about how planets actually form (see Planet formationplugin-autotooltip__plain plugin-autotooltip_bigPlanet formation

"The origin of planets is a vast, complex, and still quite mysterious subject. Despite decades of space exploration, ground-based observations, and detailed analyses of meteorites and cometary grains (the only space samples available in our laboratories),). There are also doubts about how quickly the Earth would lose its initial heat. And some research groups are suggesting that there could be as-yet-unknown nuclear processes in operation within the Earth, possibly bringing the radioactive budget up to as much as 90%.

Proposed new neutrino-detection experiments may be able to further fine-tune the proportions (ref.)

Further reading 'What Keeps the Earth Cooking?' Lawrence Berkeley National Lab, US.

* Editor's note :

Many are surprised by the 'thinness' of the Earth's crust. It varies from about 90km thick, down to 0km in some volcanic areas. To illustrate, the Science Museum in London has a soccer ball in a glass case. Stuck to the ball is a postage stamp, which roughly demonstrates the average comparative thickness of the crust.